The present invention relates to certain polyester polyols suitable for preparing polyisocyanurate foams having excellent dimensional strength and achieving ASTM E 84 Class 1 (25 Flame Spread) flame retardancy without the use of flame retardant additives. The use of Flame Spread rating data is not intended to represent large scale fire hazard resistance properties, but is used only as a reference to historical data.
The use of polyester polyols to prepare urethane foams is an old and well known procedure. Over the years many inventions have been made to improve the resultant products in terms of lower cost, low viscosity to permit ready handling in conventional urethane mixing and pumping systems, better dimensional strength of the foam, increased fire retardancy of the foam, and many other properties. While some success has been encountered in one or more of the desired properties it has not been possible to have polyester polyol resins with the requisite low viscosity and resultant foams having the desired dimensional strength, necessary fire retardancy, and low cost.
Thus, for example, it has been necessary to add fire retardants such as tri(.beta.-chloroisopropyl) phosphate to polyester polyols to obtain the necessary fire retardancy. In some instances such fire retardants also act to make the polyester polyol blend miscible with the CFC-11 (chlorofluorocarbon #11) conventionally used to make the foam or to lower their viscosity sufficiently to make them useful. Efforts to avoid the negative plasticizer effect of fire retardants by use of certain surfactants to lower viscosity and/or provide miscibility with the CFC-11 have not been successful since the resultant foams burn too readily and lack the adequate fire retardancy.
With some low molecular weight polyester polyols which have an adequate low viscosity, it has been found that they oftentimes have a high hydrogen content which produces poor fire retardancy or a hydroxyl number so high as to require a large proportion of isocyanate to form a suitable foam, thereby greatly adding to cost. Because of a low proportion of polyol, the CFC-11 must be blended into the isocyanate into an "A" blend portion as it is conventionally called. This imbalance precludes the use of normal urethane mixing equipment and creates mixing problems.
Further efforts to solve the problems of making suitable polyisocyanurate foams involve the use of a low molecular weight polyether diol or a polyether polyol resin mix with a functionality slightly above 2 as the urethane modifier to reduce the friability of the foam. These approaches have the same limitations as previously explained with respect to the use of low molecular weight polyester polyols.
It has not been possible to use existing polyols in a conventional (normal) "B" blend with a suitable amount of CFC-11 to make a low density foam (2 lbs./cu. ft. or less) without the need to dilute or blend the polyol with a suitable fire retardant to make a Class 1 (25 Flame Spread) foam.